The primary focus of this proposal is to understand the regulation and modulation of muscle contractility mammalian skeletal muscle and how it is affected by body size or scaling. The specific aims are to determine the effects of scaling on skeletal muscle fibers from mice, rats, dogs, humans, and horses1) on shortening velocity at the cellular level, 2) on in vitro motility speed , thereby eliminating the regulatory effects of the non-thick filament proteins and 3) on myosin ATPase activity,which is one of the most important regulators of skeletal muscle shortening velocity. The goal of the project is to gain a greater understanding of the role the myofibrillar proteins play in skeletal muscle contractility. The first specific aim will demonstrate the effects of scaling at the cellular level. The next two specific aims will study the underlying mechanism behind these differences by studying the effects of scaling on the various myofibrillar proteins at the molecular level. In order to accomplish the specific aims of this project, two experiments will be performed. In experiment l, shortening velocity and myosin ATPase activity will be measured in single muscle fibers from each species. In experiment 2, the in vitro motility speed will be measured in single muscle fibers from each species. The myofibrillar protein isoforms from the muscle cells used in both experiments will then be characterized by gel electrophoresis.